In recent years, Internet and mobile communication services have been spreading rapidly. Internet communication is realized by sending IP packets with use of the IP (Internet Protocol) (RFC 791), which is actually the standard of the communications via the Internet. In the case of such the Internet communication, each apparatus connected to the Internet is required to have an IP address decided uniquely in the world so as to be distinguished from others. The Internet decides the route for each IP packet according to the IP address.
However, general users of the Internet use their names (FQDN (Fully-Qualified Domain Name)) so as to specify destinations without knowing it. The DNS (Domain Name System) is used on the Internet. In the DNS are registered both IP address and name of the apparatus connected to the Internet. When an Internet user specifies a destination apparatus with the name, the DNS can search the IP address corresponding to the name. The DNS is a distributed data base. In case a DNS cannot reply to a query from a terminal, it puts a query to another DNS about the matter.
The IPv4 addresses used at present are expected to be used up soon as a result of such the rapid spreading of the Internet. In order to solve this problem, therefore, it is now under examination to employ a new system of IPv6 addresses. FIG. 19 shows such the IPv6 address system regulated by the RFC 2460. An IPv6 address 700 takes a three-layer structure (Public Topology 707, Site Topology 708, and Interface ID 706). The Public Topology 707 identifies a provider for relaying the Internet traffic and it is configured by a format prefix 701, a TLA ID 702 for denoting the top hierarchical layer of the routing, a reserved area 703, and an NLA ID 704 for denoting the second hierarchical layer of the routing. The Site Topology 708 identifies a sub-network in an end-user site connected to the Internet and it includes an SLA ID 705. The Internet ID 706 identifies the interface on the sub-network. When a terminal is connected to the Internet while the IPv6 IP address autoconfiguration function is used, the terminal can receive both Public Topology 707 and Site Topology 708 from the Internet, so that the terminal can combine them with an Interface ID and generate an IP address. Because the address autoconfiguration function can be used for the IPv6 addresses such way, the IPv6 address system can make it convenient to specify a destination apparatus with its name more than when it is specified with its IP address. This is why a destination apparatus should rather be specified with its name in the case of the IPv6 address system.
Because the IP address corresponding to an apparatus name is changed every moment, employment of another method is now under examination in the IETF (Internet Engineering Task Force); the method updates the DNS information dynamically (dynamic DNS). Concretely, the IETF RFC 2136 regulates the specifications of the message for updating the DNS dynamically.
On the other hand, the rate of data communication to the whole communication is getting larger and larger in mobile communication services in recent years. In order to improve the efficiency of such data communications, it is now under examination to employ such mobile packet communication networks as the PDC-P (PDC-Packet), the GPRS (General Packet Radio Service). While there are many communication protocols used for such mobile packet communication networks, the IP is the main stream of those protocols now. Generally, a mobile communication network is configured by a wireless access network consisting of base stations and base station controllers, and a core network consisting of subscriber nodes and gateway nodes. A mobile packet communication network formed on the basis of the GPRS method uses the signaling procedure specific to the GPRS just like the telephone communication so as to set a connection from an origination terminal to a gateway node of the home network before starting the packet communication. When the IP packet is sent from the origination mobile terminal to the destination mobile that uses a permanent IP address, a header addressed to the gateway node of the home network is added (encapsulation) to the IP packet in the subscriber node. The IP packet is then sent to the gateway node existing in the home of the origination terminal. In this case, the home gateway node is decided when the subscriber contract is made. Hereinafter, the gateway node is fixed. In this home gateway node, the added header is removed (decapsulation), thereby the original IP packet is restored. The home gateway node of the destination mobile terminal is identified by the IP address of the destination mobile terminal written in the destination address field of the original IP packet header. The IP packet is thus transferred to the node. The destination gateway node identifies the visiting subscriber node from the IP address set in the IP packet header, then encapsulates the IP packet again and sends it to the visiting subscriber node. The visiting subscriber node decapsulates the IP packet so as to restore the original IP packet and transfer the packet to the destination mobile terminal such way. Each IP packet thus arrives in the destination mobile terminal. A communication section in which an IP packet is decapsulated to the original one after an IP packet is capsulated is usually referred to as a tunnel.
In a mobile communication network, an IP address is allocated in two ways; in one way, an IP address is allocated to each mobile terminal permanently (hereinafter, to be referred to the permanent IP address method) and in the other way, an IP address is allocated to each mobile terminal when the mobile terminal begins communication so as to make the use of the IP address effective (hereinafter, to be referred to as the dynamic IP address method). An IP address allocated to a mobile terminal has the same format as that of the node to be connected to the mobile terminal permanently as usually. Although IPv6 addresses will be allocated to mobile terminals in the future, terminals that use such IPv6 addresses generally will not have permanent IP addresses. Consequently, a terminating service supplied to the mobile terminals that use dynamic IP addresses will become an indispensable function for mobile packet communication networks.
In the present IPv4 address system, almost all the user terminals have dynamic IP addresses allocated from each ISP (Internet Service Provider) when they begin communication respectively. However, to begin communication between terminals that use dynamic IP addresses, the packet network is required to have a directory service server and each terminal to have an application program for using the directory service.
On the other hand, the IETF is now examining the mobile IP specifications corresponding to the IPv6 address system. In the case of the mobile IPv6 address system, an origination terminal sends a packet addressed to a destination mobile terminal to the home address thereof, allocated by the provider. The home agent of the destination mobile terminal then receives the packet sent to the home address. Then, the home agent adds a header including the visiting address of the destination mobile terminal to the received packet, then transfers the packet to the destination mobile terminal. Receiving the packet, the destination mobile terminal sends a control signal to the origination terminal. The control signal includes the visiting address of the destination mobile terminal itself. The origination terminal then stores the visiting address information of the destination mobile terminal included in the control signal, thereby the visiting address of the destination mobile terminal can be used for the subsequent packet communications between those terminals. The visiting address mentioned above means an address allocated dynamically to a mobile terminal in the visiting network of the mobile terminal.